A typical railroad freight car includes a car body supported on a pair of wheeled trucks which are confined to roll on rails or tracks. Each truck includes a bolster extending essentially transversely of the car body longitudinal centerline. In the preponderance of freight cars, a pivotal connection is established between the bolster and railcar body by center bearing plates and bowls transversely centered on the car body underframe and the truck bolster. Accordingly, the truck is permitted to pivot on the center bearing plates under the car body. As the railcar moves between locations, the car body also tends to adversely roll from side to side.
Attempts have been made to control the adverse roll of the railcar body through use of side bearings positioned on the truck bolster outwardly of the center bearing plates. A “gap style” side bearing has been known to be used on slower moving tank/hopper railcars. Conventional “gap style” side bearings include a metal, i.e. steel, block or pad accommodated within an elongated open top pocket or recess defined on the truck bolster. An elongated and upstanding housing or cage, integrally formed with or secured, as by welding or the like, to an upper surface on the truck bolster defines the open top recess and inhibits sliding movement of the metal block relative to the bolster. As is known, a gap or vertical space is usually present between the upper surface of the “gap style” side bearing and the underside of the railcar body.
Other conventional “gap style” side bearings have included roller bearings carried for rolling movements within the elongated housing or carrier mounted on the upper surface of the railcar bolster. The roller extends above an uppermost extent of the housing or carrier and engages with an underside of the railcar body. Such side bearings are able to support the railcar body with respect to the bolster while at the same time permitting the bolster, and therefore the truck, freedom to rotate with respect to the car body as is necessary to accommodate normal truck movements along both straight and curved track.
Under certain dynamic conditions, coupled with lateral track irregularities, the railcar truck also tends to adversely oscillate or “hunt” in a yaw-like manner beneath the car body. The coned wheels of each truck travel a sinuous path along a tangent or straight track as they seek a centered position under the steering influence of the wheel conicity. As a result of such cyclic yawing, “hunting” can occur as the yawing becomes unstable due to lateral resonance developed between the car body and truck. Excessive “hunting” can result in premature wear of the wheeled truck components including the wheels, bolsters, and related equipment. Hunting can also furthermore cause damage to the lading being transported in the car body.
Track speeds of rail stock, including tank/hopper cars, continue to increase. Increased rail speeds translate into corresponding increases in the amount of hunting movements of the wheeled trucks. “Gap style” or those side bearings including roller bearings simply cannot and do not limit hunting movements of the wheeled trucks. As such, the truck components including the wheels, bolsters, and related equipment tend to experience premature wear.
The art has also contemplated constant contact side bearings for railcars. Constant contact railcar side bearings not only support a railcar body with respect to the bolster during relative rotational movements therebetween but additionally serve to dissipate energy through frictional engagement between the underside of the railcar body and a bearing element thereby limiting destructive truck hunting movements. Constant contact side bearings typically include a housing assembly including a base and a cap. The base usually has a cup-like configuration and includes at least two apertured flanges, extending in opposed radial directions relative to each other, permitting the base to be suitably fastened to the bolster. In one form, the cap is biased from the base and includes an upper surface for contacting and rubbing against a car body underside. The cap must be free to vertically move relative to the side bearing base.
Such constant contact side bearings furthermore include a spring. The purpose of such spring is to absorb, dissipate, and return energy imparted thereto during a work cycle of the side bearing assembly and resiliently position the upper surface of the cap, under a preload force, into frictional contact with the car body underframe. The spring for such side bearings can comprise either spring loaded steel elements or elastomeric blocks or a combination of both operably positioned between the side bearing base and the cap. An elastomeric block which has been found particularly beneficial is marketed and sold by the Assignee of the present invention under the tradename “TecsPak.” As will be appreciated, however, such an elastomeric block, by itself, lacks longitudinal stiffness and, thus, requires surrounding housing structure to provide added support and stiffness thereto.
There are at least two design challenges presented in connection with the design of a constant contact side bearing assembly. First, and during the course of operation, the sliding clearance between the base and cap of a constant contact side bearing assembly becomes enlarged due to abrasion and wear. Such wear is a critical performance detractor to the side bearing assembly. That is, any gap between the base and cap of the side bearing housing assembly adversely permits longitudinal or horizontal shifting movements of the cap relative to the housing thereby reducing the energy absorption capability for the side bearing assembly—a critical operating criteria for the side bearing assembly. Of course, if the gap between the base and cap of the side bearing housing assembly reaches a critical limit, the side bearing assembly is no longer useful and will be condemned.
A second design challenge involves those constant contact side bearings which use an elastomeric spring and relates to the buildup of heat in proximity to the elastomeric spring. During operation of the railcar, frictional contact between the railcar body and the side bearing assembly results in the development of heat buildup. Unless such heat buildup can be controlled, the elastomeric spring will tend to soften and deform, thus, adversely affecting the operable performance of the constant contact side bearing assembly.
The frictional sliding relationship between the side bearing assembly and the related railcar component can create a temperature within the side bearing assembly that can exceed the heat deflection temperature of the elastomeric spring thus causing the elastomeric spring to deform. As used herein and throughout, the term “heat deflection temperature” means and refers to a temperature level at the which the elastomeric spring, regardless of its composition, tends to soften and deform. Deformation of the elastomeric spring can significantly reduce the ability of the elastomeric spring to apply a proper preload force and, thus, decreases vertical suspension characteristics of the side bearing assembly which, in turn, results in enhanced hunting of the wheeled truck. Enhanced hunting and/or unstable cyclic yawing of the truck increases the resultant translation/oscillation of the railcar leading to a further increase in the heat buildup and further deterioration of the elastomeric spring.
Thus, there is a continuing need and desire for a railcar constant contact side bearing assembly having components which are designed to optimize energy absorption and related performance criteria for the side bearing assembly while inhibiting deterioration of an elastomeric spring resulting from localized heat.